The present invention relates to a hydraulic actuator preferably used for controlling braking fluid of an anti-lock braking system installed in an automotive vehicle.
In the anti-lock braking system, the hydraulic actuator increases or decreases the hydraulic pressure of each wheel cylinder to prevent the wheel from locking during the braking operation.
FIG. 6 shows a conventional hydraulic actuator disclosed in the U.S. Pat. No. 5,244,262. This actuator comprises two reservoirs 101 and 102 each storing the braking fluid released from a wheel cylinder. Four pressurizing control valves 103, 106, 107 and 110 are used for controlling the connection and disconnection between the master cylinder and wheel cylinders. Four depressurizing control valves 104, 105, 108 and 109 are used for controlling the connection and disconnection between the wheel cylinder and the reservoirs 101 and 102. Two plunger pumps 111 and 112 supply the braking fluid to the master cylinder from the reservoirs 101 and 102, respectively. Two dampers 113 and 114 suppress the pulsation flow of the braking fluid supplied from the pumps 113 and 114.
The reservoirs 101 and 102, the pressurizing control valves 103, 106, 107 and 110, the depressurizing control valves 104, 105, 108 and 109, the plunger pumps 111 and 112, and the dampers 113 and 114 are integrally assembled in a housing 115.
According to this conventional hydraulic actuator, all of the pressurizing control valves 103, 106, 107 and 110 and all of the depressurizing control valves 104, 105, 108 and 109 are disposed at a same side with respect to an axial line 116 of the plungers 111 and 112. The reservoirs 101 and 102 and the dampers 113 and 114 are disposed along a line at the other side with respect to the axial line 116. This arrangement makes it possible to effectively utilize an available space to a certain degree.
However, a width size of the housing 115 is substantially restricted by the four sequential elements, i.e., reservoirs 101 and 102 and dampers 113 and 114, arrayed in a line.
FIGS. 7A and 7B show an improved hydraulic actuator proposed in the U.S. Pat. No. 5,658,056. According to this conventional hydraulic actuator, two reservoirs 121 and 122 are disposed in parallel with two dampers 123 and 124 so that their axes are positioned at four corners of a rectangle. This arrangement is advantageous in reducing the width size of an actuator housing 125.